Linear motion guide units are extensively used in recent years with installed in the relatively sliding parts in machinery including machine tools, assembling machines, conveyors, semiconductor fabricating equipment, measuring instruments, and so on, while finding further increased applications in various industrial fields. Moreover, the linear motion guide units of the class using more than one roller, although used in diverse industrial fields, are still needed to get more in stiffness and accuracy, shrunken in size, and more adaptable to severe operating conditions including high velocity and high acceleration/deceleration in sliding movement. The design requirements as stated just earlier are needed especially in semiconductor fabricating equipment and measuring instruments.
In commonly-assigned Japanese Laid-Open Patent Application H04-194 413, a linear motion guide unit with four rows of circulating circuits for more than one rolling element is disclosed in which there are provided a lengthwise sleeve to define a return passage therein and forward and aft end caps to define turnaround passages therein, lengthwise opposite ends of the sleeve and the mating surfaces around the turnaround passages of the end caps being made in complementary to each other to come into close fit-engagement with each other. On the lengthwise opposite ends of the sleeve, there are made raised portions and recessed portions. On the mating surfaces around the turnaround passages of the end caps, in contrast, there are recessed portions to conform to the raised portions on the sleeve and raised portions to fit into the recessed portions on the sleeve. The fitted construction between the sleeve and the end caps as stated earlier helps determine with accuracy the relative location among the carriage, sleeve and the end caps to keep the return passage and the associated turnaround passages in precise alignment with each other.
The linear motion guide unit with four rows of circulating circuits constructed as stated earlier, nevertheless, has need in years of certain positively smooth rolling of the rollers through the circulating circuit even with rollers slenderer than ever to cope with recent demands for miniaturization, high operating-velocity and diverse purposes of the linear motion guide units. With the prior linear motion guide unit as recited earlier, the recessed portion on the sleeve and the associated raised portion on the end cap come into complementary mating relation with one another along three sides of a rectangle viewed in transverse section of the return passage. With the linear motion guide unit using the rollers, especially rollers small in diameter, thus, the mating construction between the sleeve and the associated end cap has to be made less in discrepancy between them, that is, made with high-precision assembly to provide the smooth circulating circuit in which the rollers are ensured to roll through there without getting stumbled on the mating construction.
Another linear motion guide unit is disclosed in Japanese Laid-Open Patent Application 2007-92 902, in which a sleeve to define a return passage is lighter in location and assembly thereof, and further effective in keeping operating performance and noise characteristics against getting worse. With the prior linear motion guide unit, the sleeve to define the return passage is beveled at longitudinally opposite ends thereof to make obliquely open ends and against each of which outside and inside halves of a spigot extended beyond an end cap are brought into close abutment against the oblique ends to form an end-to-end contact plane lying askew between the sleeve and the associated spigot halves. The return passage inside the sleeve is made circular in transverse section in conformity with the rolling element of ball.
With the prior linear motion guide unit constructed as stated earlier, the sleeve and the spigots have to come into close abutment against each other with no gap at the end-to-end contact plane between them because the sleeve is made circular in the outside contour thereof as well. Any gap at the end-to-end contact plane between the sleeve and the associated end caps would allow the sleeve turning over the gap. Some turning of the sleeve can lead to no practical problem when the balls are selected as the rolling elements. With the sleeve rectangular in transverse section to allow the rollers running through there, in contrast, any turning or angular displacement of the sleeve, albeit slightly, causes any staggered relation in rectangular return passage between the sleeve and the associated end cap, getting the circulating motion of the rollers interfered.
A further another prior linear motion guide unit, although not shown, is known in which the spigot with no split raised above the inward surface of the end cap in communication with the turnaround passage to have the right flat extremity extends somewhat into a longitudinal bore inside the carriage to come into direct abutment against a sleeve, thereby connecting the sleeve with any one opening of the turnaround passage made in the end cap, the opening coming face-to-face with the return passage. With the connecting construction as stated just earlier, the guide surface extending out of the inward surface of the end cap to carry the axially opposite ends of the roller comes into direct abutment against the associated end of the sleeve over the entire width thereof. Thus, even if there is any gap between the sleeve and the associated spigot, the rollers are apt to stumble on the gap when rolling across the connecting construction to lean or topple over to fall into the gap.
In the linear motion guide units in which the rollers or needles are selected as rolling elements, the rollers have to be guided not only on their circular rolling surfaces, but also on their axially opposite end surfaces to run through the circulating circuit in good rolling order without leaning in rolling posture. Especially, the rollers small in diameter or needles are easy to encounter any obstacle at even tiny gap lying on a butt joint between the return passage and the associated turnaround passage. Thus, it still remains a significant challenge to ensure smooth, steady rolling motion of the rollers throughout the circulating circuit.